1. Field of the Invention
This invention concerns a light wave distance meter comprising an analog light source which directs modulated light emitted by an optical device at a target, and measures the distance to the target by detecting the modulated light reflected from the target.
2. Prior Art
FIG. 5 shows a light wave distance meter as known in the prior art.
In FIG. 5, 1 is a support free to rotate about a vertical axis on a base 2. A telescope unit 4 housing a telescope 3 is supported by this support 1 such that the unit is free to rotate about a horizontal axis. As shown in FIG. 8, the telescope 3 is provided with a prism 8 and a line-of-sight system, not shown.
The prism 8 splits modulated light emitted by the optical device 5 into measuring light and reference light, directs the measuring light at a target (corner cube) 6, and guides the light reflected by the corner cube 6 and the aforesaid reference light to a photodetector 7.
FIG. 6 is a block diagram showing the signal processing circuit of the light wave distance meter. In FIG. 6, 11 is a circuit which causes modulated light to be emitted from the optical device 5, 12 is a photodetector circuit which performs a phase comparison between the reference light and reflected light from the signal output of the photodetector 7, 13 is a digital circuit which controls the light emitting circuit 11 and computes the distance to the corner cube 6 from the phase difference of the photodetector circuit 12, and 14 is a power supply circuit.
In order to make the light wave distance meter more compact, a light emitting part (analog photoemitter) 17 comprising the optical device 5 and light emitting circuit 11, a light receiving part (analog photodetector) 18 comprising the photodetector 7 and photodetector circuit 12, and the power supply circuit 14, are disposed in the lower housing 4a which is the lower part of the telescope unit 4 (below the lens barrel, not shown, of the telescope 3), while the digital circuit 13 is disposed in the upper part 4b of the telescope unit 4 (above the lens barrel, not shown, of the telescope 3).
The precision of the distance meter is however largely affected by optical noise and electrically induced noise. To prevent optical noise, the inside of the telescope is painted black so that it does not reflect light, and a partition is provided in the center so that the measuring light does not interfers with the reflected light.
Further, electrically induced noise is reduced by shielding the apparatus with a metal cover or by providing a metal sheet partition between the light emitting part 17 and the light receiving part 18.
In this conventional light wave distance meter, however, high current and high voltage circuitry is used in the light emitting circuit 11 and the light receiving circuit 12. Intense noise is therefore generated by the light emitting part 17, and as the light emitting part 17 and light receiving part 18 are arranged adjacent to each other in the lower housing 4a, the light receiving part 18 is affected by this noise. Using only the aforesaid metal cover shield or metal sheet partition, it was thus impossible to reduce the noise sufficiently.